Circuits for thermionic tubes



May 27, 1930 F. s. MCCULLQUGH 1,759,910

-- CIRCUITS FOR THERMIONIC TUBES Filed Aug. 25', 1925 I I ATiORNEY character,

' ating from the circuit Patented May 27, 1930 UNITED STATES,

PATENT, OFFICE,

FREDERICK S. MGCULLOUGH, OF EDGEWOPD, PENNSYLVANIA CIRCUITS FOR THERMIONIC TUBES Application filed August 25, 1925. Serial No. 52,307.

This invention is for an improvement in a circuit for thermionic tubes, particularly tubes of the type having an indirectly heated cathode, and is especially applicable to a radio receiving circuit.

Certain formsof tubes have beendesigned having a cathode which is heated by means of a suitable heater adapted to be included in an alternating current circuit. Suchtubes, as usually constructed, have in addition to the usual plate and grid, a cathode in the form of a cylinder fitted 0 :er a porcelain rod. Within this rod, is aresistance wire adapted to be coupled in an alternating current circuit.

Since the cathode is not energized by the passage of current therethrough, as is the cathode of the more common type of tube which has the catliode inthe form of a filament, it is termed an equi-potential cathode, and there is in the normal operation of such tubes, as now used, no means for obtaining a potential gradient along the cathode. Such a tube, furthermore, is not capable of delicate temperature variations, and it is not convenient to control the temperature of the cathode. It is accordingly quite difficult to control the oscillations of the' tube, particularly as a detector.

The present invention has for its object to provide in combination with a tube of this means for establishing a potential difference between different parts of the cathode and then varying this potential on different parts of the cathode as required, to control the oscillations of the tube.

A further important object of the invention is to provide means in the circuit for elimineither partly or entirely, the hum which might otherwise exist by reason of the use of alternating current for energizing the cathode.

The invention may be readily understood by reference to the accompanying drawing which shows a tube of the type referred to coupled into a circuit embodying my inven-= tion. It will be understood thatthe figure shown is more or less diagrammatic, and that the invention is not restricted to the particular construction of the tube or the arrangement of the circuits therein illustrated.

In the drawing, 5 designates a suitable vessel within which is a plate or anode 6. *Within 2 the plate is a grid 7 of any preferred construction. Within the grid and out of contact therewith, is the cathode. The cathode as herein illustrated, comprises two convergently arranged refractory rods 8 through which passes a. V-shaped resistance wire 9. Fitted over each rod 8 is a cylindrical shell 10., These shells form the cathode of the tube. They are preferably out of contact with each other, and are either of a substance having a high electron emissitivity, or they are coated with a suitable substancefor giving the desired emission. The opposite ends of the V-shaped reslstance wire 9 may be coupled in circuit with a suitable sourceofalternating current as indicated at 11. In practice, this alternating current maybe derived from a house lighting circuit stepped down by a suitable transformer, as indicated. -Leading through the tube, at a suitable point, is a conductor 6 which connects the plate with a suitable ex ternal circuit. This circuit may include the translating device T, which may be a telephone or any other suitable current responsive device and a source of plate current, in-

dicated at B- A wire 7 leads from the gridof the tube. One of the cathode members 10 is connected with a lead-in wire 12, and the other one with a lead-in wire 13.

Wire 13 may beconnected to the plate and grid circuits in a suitable manner, this point of connection being indicated at 16. Interposed between point 16 and the cathode of the tube, is a suitable reactance or impedance designated 14. This may be shunted by a suitable radio frequency by-pass, as forinstance, a condenser 14*. Between the point 16 and the reactance 14 there is preferably a resistance 17 that connects wires 12 and 13. This is preferably a variableresistance as shown. It may for example, have a maximum resistance of 25,000 ohms and this may be varied as desired. The variable contact for the resistance is indicated at. 17 and is connected with wire 12. If, desired, a second reactance may be included in wire 12 between the contact 17 and the portion of the cathode with which wire 12 connects, this impedance 25 of the tube.

30 wire. The by-pass condenser 14 55 havin so tions of the cathode,

being shown in dotted lines and being desig nated 15.. It, too, may be shunted by a radio frequency by-pass as indicated in dotted lines. Under some circumstances it might be 5 desirable to provide additional condensers as indicated in dotted lines at 21.

The grid circuit may be of any well known character; I have shown the circuit as a detector circuit with an inductance 20 and a.

grid leak and grid condenser diagrammatically indicated at 19.

In the operation of the tube, the resistance wire 9 is heated by an alternating current passing therethrough. This in turn heats the porcelain rods 8 and the cathode members 10.

When the cathodes are heated, there will be a current flow between the plate and the oath odes as in any thermionic tube. By adjustment of resistance 17 it is possible to restrict 20 the current flow between one of the cathodes and the plate, so as to create a potential V difi'erence between the two cathode members. By proper adjusment of this potential dif frence it is possible to control the oscillations At the same time the impedance 14 between the cathode and the resistance 17 functions to retard alternating current impulses set up in the circuit by reason of the use of an alternating current in the heater will permit radio frequency oscillations to pass the imedance but the lower alternating current Frequencies will be substantially eliminated. The capacity and arrangement of the condensers and the nature of the impedances may be adjusted and varied according to the characteristics of the circuit in which the tube is used and according to the manner in'which it is desired that the tube shall function. It 40 will accordingly be seen that by the adjustment of resistance 17, the tube no longer neces'sarily has the characteristics of an equipotential cathode "tube, and that conditions corresponding to those existing in a tube having a'directly resistance heated cathode can,

in effect, be duplicated.

claim as my invention: 1. Thec'ombination with a thermionic tube ,haii ng acathode comprising a plurality of sebtion's and a heater for the cathode, of wires couiiactdflwich the difl'erent sections of the ca'thbilefandja variable resistance connected acrossjsaid wires.

"2. The combination with a thermionic tube g a cathode comprising "two leg portions and a heater for two leg portions of the cathode, of wires connected with the difierentleg portions of the cathode, a variable resistance connected between said leg porand a reactancebetween the cathode and the resistance.

3. The combination with a thermionic tube having a cathode including two separate members, m'eansincluding a resistancewire I5 for heating said cathode members, a wire in one of between between the leading from each of said cathode members, and a vanable reslstance connected across said wires.

4. In a radio circuit, a combination with a thermionic tube having a cathode and a heater for the cathode, and having a plate and a grid, of a plate circuit and a, grid circuit, a pair of wires leading from separate points'of the cathode,-one of said wires being connected to theplate and grid circuits, and a resistance connected across said wires.

V 5. A. radio "circuit as claimed in claim 4 characterized by the provision of a reactance said. wires interposed between the cathode and the resistance, said reactanc'e being shunted by a radio frequency by-pass.

6. The combination with a'thermionic tube having a cathode formed of two separate members, means for heating said' cathode members, a wire leading from each of said cathode members, and a variable resistance connected across said wires constituting the only conductive connection between the two separate members of the cathode.

7 The combination with a thermionic tube, a cathode comprising a pair of separated cathode members, means for heating said cathode members, of said cathode necting the two one of the wires.

8. 'In a radio with a thermionic tube having a pair oi :cathode members therein, means for heating the cathode members, said tube having also a grid and plate, of a grid circuit and a plate circuit connected with a grid and plate respectively, a wire leading from one of the cathode members connected with both the grid and plate circuits, a wire leading from the othercathode member connected with the first member through a variable resistance,

members, a resistance conwlres, and an impedance in the resistance and the cathode mem her to which it connects.

9. The combination with a thermionic tube having a divided cathode, an alternating current heater for the cathode, an external impedance in circuit with one of the divided portions of the cathode, and a variable resistance through which the divided, portions of the cathode are connected.

10. The combination with a thermionic tube having a cathode, an alternating current heater for the cathode, and an external impedance in circuit with the cathode for reducing current variations resulting from the accircuit, the combination and an impedance in one of the wires,

a wire leading from each i tions of the alternating current heater, and

a radio frequency by-pass shunted around the impedance.

11. The combination with a radio tube having a pair of cathodes heated by alternating current, of a resistance connecting the two cathodes,

resistance and one o the and an impedance inter osedcathodes, said impedance being shunted by a capacity for by-passing radio frequency currents. Y o g 12. The combination with. a thermionic tube having a cathode consisting of a pair of electron emitting members, heating means including a resistance wire for rendering said electron emitting members active, and a variable resistance connected between said 10 electron emitting members.

In testimony whereof I hereunto aifix my signature.

FREDERICK s. MQGULLOUGH. 

